Read the Wiki file on adding comets to the ssystem.ini file. Downloaded the Comet ZIP files. What I didn't see there was a way to add Comet ISON. Can this be done?
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Try adding this to the ssystem.ini file
parent = Sun
orbit_Inclination = 61.8916
coord_func = comet_orbit
orbit_Eccentricity = 1.000004
orbit_ArgOfPericenter = 345.5129
absolute_magnitude = 7
name = C/2012 S1 (ISON)
slope_parameter = 3.2
lighting = false
tex_map = nomap.png
color = 1.0, 1.0, 1.0
orbit_AscendingNode = 295.7319
albedo = 1
radius = 5
orbit_PericenterDistance = 0.012499
type = comet
orbit_TimeAtPericenter = 2456625.29388889
and search for it in stellarium as C/2012 S1 (ISON)
Note:- These parameters may be out of date there could be a later set of orpital parameters